1. Field of the Invention
The present invention relates to methods and apparatuses for characterizing or evaluating the strength or effectiveness of oil based drilling fluids for use in drilling wellbores in subterranean formations. The invention relates particularly to methods and apparatuses for characterizing water-in-oil or invert emulsion fluids for use in drilling wellbores in hydrocarbon-bearing subterranean formations.
2. Description of Relevant Art
The Electrical Stability meter (ES), such as the FANN 23D (FIG. 3) available from Halliburton Energy Services, Inc. in Houston, Tex., has been used to characterize invert emulsion oil-based drilling fluids for many years. The first patent issued for an ES meter was U.S. Pat. No. 2,859,404, issued Nov. 4, 1958 to B. C. Crittendon. When diesel-based drilling fluids were the dominant oil-based fluids, the ES meter was a particularly useful diagnostic tool. The higher the ES voltage measured for the fluid, the stronger or harder to break would be the emulsion created with the fluid and consequently the better the fluid would likely be as a drilling fluid.
Invert emulsion oil-based drilling fluids are stabilized by the addition of emulsifiers. The water phase is usually about 5% to about 40% of the total liquid volume and is usually comprised of (but not limited to) calcium chloride brine. The brine concentration is adjusted to control the water exchange between the drilling fluid and the formation being drilled and to aid in formation stability. If the formulation of the invert emulsion drilling fluid becomes unbalanced, due to, for example (without limitation), contamination, improper product additions, or thermal degradation, the emulsion can flip. When such flipping happens, the water becomes the continuous phase, and can the cause the solids in the drilling fluid to become water wet. Such wetting requires expensive remedial action and can lead to the loss of the well.
The strong electric field induced by the ES meter causes tiny water droplets in the drilling fluid to orient and coalesce to form conductive bridges between the faces of the electrodes of the ES meter probe. Solid materials in the drilling fluid may aid or interfere with formation of the conductive bridge. (See SPE/IADC 16077, “Investigation of the Electrical Stability Test for Oil Muds”, A. Ali, D. D. Schmidt and J. Harvey III, (1987)). These solid materials typically include, for example, weight material that is added to increase the density, drill cuttings or drilled formation solids, and colloidal solids added to control the viscosity and suspension properties of the drilling fluid.
The electrical stability test method specified by the American Petroleum Institute is “Recommended Practice Standard Procedure for Field Testing Oil-Based Drilling Fluids”, API Recommended Practice 13B-2, Third Edition, February 1998 (“the API Procedure”), incorporated herein by reference. Paragraph 8.1.1 states: “The electrical stability (ES) of an oil-based drilling fluid mud is a property related to its emulsion stability and oil-wetting capability. ES is determined by applying a voltage-ramped, sinusoidal electrical signal across a pair of parallel flat-plate electrodes immersed in the mud. The resulting current remains low until a threshold voltage is reached, whereupon the current rises very rapidly. This threshold voltage is referred to as the ES (“the API ES”) of the mud and is defined as the voltage in peak volts-measured when the current reaches 61 μA.” This API Procedure also specifies the other instrument parameters that must be controlled to make repeatable measurements. Such parameters include the waveform, AC frequency (340±10 Hz), voltage ramp rate, break current and electrode diameter and spacing (gap).
The ES meter has evolved over the years since the first instruments were introduced. Additions such as automatic voltage ramps, superior electronics and electrode design improvements have been made. However, the composition of drilling fluids has also evolved. Increasingly, oil-based drilling fluids formulated using mineral oils, synthetics (or synthetic oils), and esters instead of diesel oil are being used. ES voltages that have been good indicators of emulsion performance with diesel oil-based fluids are unobtainable with many of these newer or alternative invert emulsion based fluids. ES voltages for such invert emulsion based fluids currently range from about 100 V to over about 1000 V. Thus, with such a broad “acceptable” range, ES measurements with an ES meter no longer provide the precise or dependable information needed for practical judgments regarding emulsion performance, quality or stability.